1. Field of the Invention
The present invention relates to all fields where a signal is processed.
2. Description of the Prior Art
FIG. 11 is a circuit diagram illustrating an example of a conventional electronic volume circuit.
A ladder resistor 4 comprises resistors R.sub.1, R.sub.2, . . . , and R.sub.n. A group of analog switches 3 comprises analog switches S.sub.1, S.sub.2, . . . , and S.sub.n. The group of analog switches 3, together with the ladder resistor 4, forms an electronic volume 100. Supply voltages +V.sub.CC and -V.sub.CC are supplied to operational amplifiers 1 and 2. Resistors 5 and 6 are connected with the operational amplifiers 1 and 2, respectively. The resistors 5 and 6 determine potential level of DC component of input signals to be fed to the operational amplifiers 1 and 2, respectively. The operational amplifiers 1 and 2 serve as buffer amplifiers for the electronic volume 100.
Capacitors 7, 8 and 9 are provided for AC coupling.
When this electronic volume 100 is employed in an audio circuit, a signal to be processed therein usually oscillates between -7 V and +7 V. Therefore, positive supply voltage +V.sub.CC is set at +7 V, while the negative supply voltage -V.sub.CC is set at -7 V. The supply voltages +V.sub.CC and -V.sub.CC are also given to the group of analog switches 3.
FIG. 12 conceptually illustrates waveforms representing signals to be processed in the audio circuit. The signals oscillate around the axis at a ground level (GND level), i.e., 0 V which is a median value between supply voltages +V.sub.CC and -V.sub.CC. A waveform W.sub.1 represents a signal which has a relatively small amplitude, while a waveform W.sub.2 represents a signal which has an approximately maximum amplitude. Even if a signal has a maximum amplitude (from -7 V to +7 V), some amount of the voltage is consumed in the buffer amplifiers 1 and 2. When about 1 V of the voltage is consumed, the maximum amplitude of the signal turns to 12 V.sub.P-P as represented by the waveform W.sub.2.
The operational amplifiers 1 and 2 generally has a bipolar semiconductor structure. It is well-known that this structure has an excellent performance capacity. Since the bipolar semiconductor structure generally provides from 16-volt to 20-volt breakdown voltage, the bipolar semiconductor structure can be employable in the audio circuit which requires breakdown voltage of about 14 V, i.e., the difference between the positive and the negative supply voltages.
The analog switches S.sub.1, S.sub.2, . . . , and S.sub.n, on the other hand, preferably have a CMOS (complementary metal oxide semiconductor) structure. An analog switch having the CMOS structure can provide a superior performance which will be more fully described later. Breakdown voltage of the CMOS structure is generally about 8 V. Therefore, when the required breakdown voltage is about 14 V as is in the case of the audio circuit, there is a need for conducting an extra process for obtaining high breakdown voltage such as a step of enlarging dimensions of elements, for example.
Such an extra process complicates the whole processes of integrating the operational amplifiers 1 and 2 and the electronic volume 100 to thereby produce a BiCMOS (bipolar complementary metal oxide semiconductor) structure.